1. Field of the Invention
This invention relates to a method for the purification of ethylene oxide. More particularly this invention relates to a method for the purification of ethylene oxide by the steps of introducing the gas formed by catalytic gas-phase oxidation of ethylene with a molecular oxygen-containing gas and consequently containing ethylene oxide into an ethylene oxide absorber and led into counterflow contact therein with an absorbent liquid, circulating the gas emanating from the top of the ethylene oxide absorber to the step for oxidation of ethylene, supplying the ethylene oxide-containing bottom liquid of the ethylene oxide absorber to an ethylene oxide stripper, allowing the ethylene oxide stripper to obtain ethylene oxide through diffusion via the top thereof, condensing the resulting distillate containing ethylene oxide and water, separating the water from the distillate in a dehydrator, separating a more volatile component from the distillate in a light ends stripper, and subsequently rectifying the remaining ethylene oxide in an ethylene oxide refiner, which method is characterized by economizing the energy for heating the ethylene oxide refiner and the light ends stripper.
2. Description of Prior Art
Ethylene oxide is generally purified as follows. The catalytic gas-phase oxidation of ethylene with a molecular oxygen-containing gas over a silver catalyst produces a reaction product gas containing ethylene oxide. This gas is led to an ethylene oxide absorber and brought into counterflow contact therein with an absorbent liquid having water as a main component thereof so as to effect recovery of an aqueous ethylene oxide solution. Then, this aqueous solution is forwarded to an ethylene oxide stripper and, by heating the bottom of the stripper, is enabled to obtain ethylene oxide through diffusion. The aqueous solution which now contains substantially no ethylene oxide is withdrawn via the bottom of the stripper to be used again as an absorbent liquid. The diffusate departing from the top of the stripper and containing ethylene oxide, water, carbon dioxide, inert gases (nitrogen, argon, methane, ethane, etc.), low-boiling impurities such as formaldehyde, and high-boiling impurities such as acetaldehyde and acetic acid is purified by being passed through the step of dehydration, the step of separation of more volatile components, and the step for separation of heavy-duty components, to give rise to ethylene oxide. Several methods for purification of ethylene oxide have been proposed. (Refer, for example, to U.S. Pat. Nos. 3,165,539; 2,771,473; 4,028,070; 3,097,215; 3,217,466; 3,745,092; 3,729,899; 3,766,714; and 3,964,980.)
The method heretofore known to the art will be described specifically below.
With reference to FIG. 1, ethylene is subjected to catalytic gas-phase oxidation with a molecular oxygen-containing gas in the presence of a silver catalyst to produce a reaction product gas containing ethylene oxide. This gas is passed through a conduit 1 and fed to the lower part of an ethylene oxide absorber 2 in the form of a packed tower or a tray tower. An absorbent liquid is introduced via a conduit 3 into the upper part of the ethylene oxide absorber 2 and brough into counterflow contact in the tower with the reaction product gas to recover not less than 99% by weight of the ethylene oxide present in the reaction product gas. Such gases as the portion of ethylene oxide which has escaped being absorbed, oxygen, carbon dioxide, inert gases (nitrogen, argon, methane, and ethane), aldehydes, and acidic substances departing from the top of the ethylene oxide absorber 2 are forwarded via a conduit 4 and circulated to the step of carbon dioxide absorption and/or the step of oxidation. In this step of absorption, such low-boiling impurities as formaldehyde and such high-boiling impurities as acetaldehyde and acetic acid which are formed in the step of oxidation of ethylene besides ethylene, oxygen, carbon dioxide, and inert gases (nitrogen, argon, methane, and ethane), let alone ethylene oxide, are absorbed all at once in their substantial proportions. The bottom liquid of the ethylene oxide absorber 2 is passed through a conduit 5 to a heat exchanger 6, there to exchange heat with the bottom liquid of an ethylene oxide stripper and gain in temperature to 70.degree. to 110.degree. C. The hot bottom liquid is then passed through a conduit 7 to a gas-liquid separation tank 8. The more volatile portion of inert gas containing ethylene oxide and water partly is separated via a conduit 9. The absorbent liquid left behind after the more volatile gas has been expelled by flushing is passed through a conduit 10 and introduced to the upper part of an ethylene oxide stripper 11 kept under top pressure of 0.1 to 2 kg/cm.sup.2 G at a top temperature in the range of 85.degree. to 120.degree. C. and heated in a conduit 13 with a heating medium such as steam or a heat medium (produced by The Dow Chemical Company and marketed under trademark designation of "Dowtherm") circulated through a reboiler 12 annexed to the ethylene oxide stripper 11 or heated directly by feeding steam to the bottom of the ethylene oxide stripper 11. As the result, not less than 99% by weight of the ethylene oxide contained in the absorbent liquid is obtained through diffusion. Part of the bottom liquid of the ethylene oxide stripper containing substantially no ethylene oxide and having a temperature of 100.degree. to 150.degree. C. is withdrawn via the bottom of the ethylene oxide stripper 11 and forwarded via a conduit 15 to the heat exchanger 6, there to exchange heat with the bottom liquid of the ethylene oxide absorber 2. The bottom liquid consequently deprived of heat is passed through a conduit 16 and further cooled by a cooler 17 having cooling water circulated through conduits 18 and 19 therein. Then, fresh water is introduced via a conduit 21 for the purpose of adjusting the ethylene glycol concentration in the absorbent liquid. The absorbent liquid is replenished with aqueous potassium hydroxide solution when necessary for the adjustment of the pH of the liquid. For the adjustment of the anti-foam agent concentration in the absorbent liquid, an anti-foam agent may be introduced into the ethylene oxide absorber 2. To prevent the ethylene glycol by produced in the hydrolysis of ethylene oxide and water, such low-boiling impruities as formaldehyde, and such high-boiling impurities as acetaldehyde and acetic acid from increasing in the absorbent liquid between the step for oxidation of ethylene with molecular oxygen and the step for stripping of ethylene oxide, the bottom liquid of the ethylene oxide stripper 11 is withdrawn via conduits 14 and 22 through the bottom of the ethylene oxide stripper 11 and forwarded to the step for concentration of the by-produced ethylene glycol.
In the meantime, the vapor containing ethylene oxide obtained via the top of the ethylene oxide stripper 11 is forwarded via a conduit 23 to a condenser 24 having cooling water circulated through conduits 25 and 26 therein. The condensate consequently produced is returned via a conduit 27 to the top of the ethylene oxide stripper 11 and the vapor which has escaped being condensed is introduced via a conduit 28 to a dehydrator 29.
The vapor is heated by either being passed through a conduit 31 which is kept heated with a heating medium such as steam or Dowtherm by a reboiler 30 connected to the dehydrator 29 or being directly heated owing to the introduction of steam to the lower part of the dehydrator 29. The water containing no ethylene oxide is withdrawn via a conduit 32 from the bottom of the dehydrator 29.
From the top of the dehydrator 29, the vapor containing ethylene oxide is forwarded via a conduit 33 to a condenser 34 having cooling water or brine circulated through conduits 35 and 36 therein. The condensate consequently formed is returned via a conduit 37 to the top of the dehydrator 29. The vapor which has escaped being condensed in the condenser 34 is introduced via a conduit 39 to an ethylene oxide vent-scrubber (not shown). The remaining part of the condensate in the condenser 34 is introduced via a conduit 38 to a light ends stripper 40.
The condensate is heated by being passed through a conduit 42 kept heated with a heating medium such as steam or Dowtherm by a reboiler 41 which is annexed to the light ends stripper 40. From the top of the light ends stripper 40, the vapor containing ethylene oxide is forwarded via a conduit 43 to a condenser 44. The condensate consequently formed is returned via a conduit 47 to the top of the light ends stripper 40. The vapor which has escaped being condensed is introduced via a conduit 48 to an ethylene oxide vent-scrubber (not shown) for the recovery of ethylene oxide.
In the meantime, from the bottom of the light ends stripper 40, the ethylene oxide separated from the more volatile component is introduced via a conduit 49 to an ethylene oxide refiner 50.
The bottom liquid is heated by being passed through a conduit 59 kept heated with a heating medium such as steam or Dowtherm by a reboiler 58 which is annexed to the ethylene oxide refiner 50. A steam of pressure of 0.5 to 3.0 kg/cm.sup.2 G is introduced via a conduit 59 to the reboiler 58 annexed to the ethylene oxide refiner 50. Then, rectification is carried out with the bottom temperature of the ethylene oxide refiner 50 maintained at 35.degree. to 85.degree. C. and the bottom pressure of the tower maintained 1.2 to 8.2 kg/cm.sup.2 G. The ethylene oxide vapor of the top temperature of 29.degree. to 81.degree. C. and the top pressure of 1.0 to 8.0 kg/cm.sup.2 G was withdrawn via the top of the ethylene oxide refiner and forwarded via a conduit 51 to a condenser 52, there to be liquefied. Part of the liquefied ethylene oxide is passed through a conduit 56 and introduced as a reflux liquid to the top of the ethylene oxide refiner 50. The remaining part of the liquefied ethylene oxide is withdrawn via a conduit 57 as an ethylene oxide product.
The vapor which has escaped being condensed in the condenser 52 of the ethylene oxide refiner 50 is introduced via a conduit 55 to the ethylene oxide vent-scrubber (not shown) for recovery of ethylene oxide.
The bottom liquid of the ethylene oxide refiner 50 is withdrawn via a conduit 67 when necessary for the separation of heavy-duty fractions of such high-boiling impurities as acetaldehyde, water, acetic acid etc.
The method for the purification of ethylene oxide described above, however, is not satisfactory in terms of the recovery of the heat of condensation of the vapor obtained through the top of the ethylene oxide stripper and recovery of the thermal energy possessed by the liquid withdrawn through the bottom of the ethylene oxide stripper. Thus, this method has entailed the disadvantage that a large volume of heat is wastefully discharged from the system. The conventional method has imposed the rule of causing the bottom liquid of the ethylene oxide stripper which has a temperature of 100.degree. to 150.degree. C. to exchange heat with the bottom liquid of the ethylene oxide absorber thereby effecting recovery of heat and thereafter cooling the bottom liquid and reclaiming the cooled bottom liquid as an absorbent liquid for use in the ethylene oxide absorber. Further, the method for the purification of ethylene oxide has entailed the disadvantage that the heating carried out in the ethylene oxide refiner consumes a large volume of heating steam.
As the result of our research to saving energy in the above process for purification of ethylene oxide, we have eventually found that the energy possessed by the bottom liquid of the ethylene oxide stripper and possessed by the top vapor thereof can be utilized effectively.
An object of this invention, therefore, is to provide a novel method for the purification of ethylene oxide.
Another object of this invention is to provide a method for the purification of ethylene oxide, which promotes effective utilization of the energy of the top vapor of an ethylene oxide stripper and/or that of the bottom liquid thereof.